Polycrystalline silicon (polysilicon) films deposited on silicon substrates using low pressure chemical vapor deposition (LPCVD) techniques have electrical and mechanical properties which are attractive both for the formation of microelectronic devices and for the production of certain types of micromechanical devices. For example, LPCVD polysilicon structures may be used in such diverse applications as micromechanical pressure sensors, as described in co-pending U.S. patent application Ser. No. 855,806, X-ray mask blanks, and single crystal on insulator material obtained by recrystallization of polysilicon deposited on the insulator.
LPCVD polysilicon films are typically deposited by pyrolytic decomposition of silane gas at low pressure, although other reaction methods are available. The equation for the chemical reaction is: EQU SiH.sub.4 =Si+2H.sub.2
This reaction takes place at an elevated temperature in an air-tight furnace, such as a modified oxidation furnace. Typically, the substrate material on which the polysilicon is to be deposited is placed in a cradle in the center of a quartz process tube which can be heated in the furnace. The tube is constructed or fitted with special doors to allow the tube to be evacuated to a moderately low pressure. Silane gas is then introduced at one end of the tube and thermally decomposes on the walls of the tube and the substrate material in the cradle, leaving thin films of polycrystalline silicon on these exposed surfaces. When the desired film thickness is obtained, the silane gas flow is discontinued and various purging cycles are carried out before the substrate materials with the deposited film thereon are removed from the furnace. The four major steps of LPCVD polysilicon depositions are (1) transport of silane, (2) adsorption of silane onto the substrate surface, (3) surface reaction of the silane, and (4) desorption of hydrogen. Silane flow rates are determined by the pumping speed of the pump, the pressure in the process tube, and the volume of the tube. Low pressures are used to enhance the uniformity of the film thickness. At sufficiently high pressures, silane gas can decompose in the gas phase to homogeneously nucleate and grow single crystal grains of polysilicon. At the lower pressures used in the LPCVD process, the silane decomposes catalytically on the substrate surface. Adsorbed silane molecules have some degree of surface mobility before the hydrogen atoms detach from the silicon, and are desorbed, either as atomic or molecular hydrogen, and are pumped away.
The initial silicon atoms also have a certain degree of surface mobility. The adatoms form islands and eventually coalesce to form the lower layer of the film. Additional adatoms therefore deposit on a surface which is chemically different then the initial surface. Surface contamination or poor substrate preparation can lead to local nucleation and growth of grains at sites on the substrate. If the growth is rapid, nodules and protrusions occur in the film, resulting in a haze or a roughness on the film surface, and in more severe cases, resulting in visible defects.
Once nucleation of single crystal silicon grains take place, growth becomes more rapid. Normal film growth in this case will result in columnar type grain structures resulting as the grains grow in the direction of the film, with consequent preferential crystal orientations. Alteration of the deposition conditions can be made to inhibit nucleation and growth, thereby reducing surface roughness, preventing preferential orientations from occurring, and resulting in a film with a lower defect density. Accelerated nucleation and growth due to surface contamination become less severe. One approach to achieving such films is to deposit fine grained, nearly amorphous silicon using lower deposition temperatures for the decomposition of silane. See, T. I. Kamins, et al., "Structure and Stability of Low Pressure Chemically Vapor-Deposited Silicon Films," J. Electrochemical Society: Solid State Science and Technology, June 1978, pp. 927-932; G. Harbeke, et al., "Growth and Physical Properties of LPCVD Polycrystalline Silicon Films," J. Electrochemical Society: Solid State Science and Technology, Vol. 131, No. 3, Mar. 1984, pp. 675-682.
Semiconductor devices typically have a variety of deposited and/or chemically induced thin films, often including films formed by low pressure chemical vapor deposition. These films exhibit strain fields which are sensitive to processing conditions, potentially resulting in internal stresses within the films, under some conditions, of several thousand atmospheres. Consequently, electronic characteristics which depend on life-times, surface states, or bulk and surface mobility are affected by the character of the strain field in the film because each of these quantities is influenced by high stress. If the strains induced in the films are ignored during processing, large or undesirable tolerances result, and in extreme cases, which are typical in very large scale integration (VLSI) device processing, device failure is a consequence. Conversely, if the problem of strain is understood, the resulting strain in the films can be used to improve device performance. For example, mobility increases due to tensile strain can and have been used to improve the performance of N-channel transistors. Thus, if microelectronic devices are to be formed on deposited polysilicon films, the control of the strain fields in the film are crucial to performance of the devices formed on the films.
Similarly, for micromechanical structures using thin polysilicon films, the design of micromechanical components, which typically involves the calculation of beam and plate deflections, requires accurate measurements and control of mechanical quantities such as the built-in strain field in the films, Young's modulus, the Poisson ratio, and the tensile strength of the film. Experience with pressure transducers and fully supported plates as used in X-ray mask blanks, and in the actual processing of various thin films, leads to the conclusion that the dominant mechanical quantity for thin films is the built-in strain field. The tensile strength is also important but secondarily so, and knowledge of Young's modulus and the Poisson ratio are needed if mechanical device performance is to be calculated--for example, for specific device design rather than feasibility studies.
The effect of the built-in strain field on the micromechanical properties of a deposited film are complex but have readily observable and significant consequences in free standing microstructures such as beams and membranes. A doubly supported beam will tend to buckle if it is sufficiently long and if the strain field is large and compressive, whereas a sufficiently short beam in the same compressive field will remain straight. Similarly, a sufficiently large membrane supported at its edges will collapse if it has a sufficiently large built-in compressive strain.
However, doubly supported beams and edge supported membranes can be made much larger if in tensile strain since the built-in strain tends to hold these structures taut. To the extent that such strain fields are inherent in the film produced by a certain process, these strain fields thus impose limits on the dimensions of the micromechancal structures that can be successfully produced. In addition to limiting the potential size of such structures, the strain fields encountered in the films can affect the desired mechanical properties of the structures such as the resonant frequencies of a thin film membrane used as a pressure transducer or microphone.
LPCVD deposited silicon films, both amorphous and polycrystalline, exhibit large compressive strain as deposited. It has been possible to reduce the built-in strain compressive strain, in some cases to very low levels, by annealing. See Harbecke, et al., supra; and R. T. Howe et al., "Stress in Polycrystalline and Amorphous Silicon Thin Films," J. Applied Physics, Vol. 54, No. 8, Aug. 1983, pp. 4674-4675. However, even after annealing such films have heretofore never exhibited tensile strain.